Autonomic damage indication in coatings

ABSTRACT

Autonomous detection of damage in a polymer coating is described by utilizing microcapsules in a polymer coating having free and/or residual amines. The microcapsules contain a color indicator, such as 2′,7′-dichlorofluorescein (DCF), bromophenol blue (BPB) or fluorescamine, which is reactive with the free and/or residual amines present in the polymer coating. For coatings without the presence of free and/or residual amines, a color indicator microcapsule can be combined with a second type of microcapsule filled with a base. When sufficient damage is inflicted to the coating, the microcapsules in and/or around an area of the damage will rupture, and the color indicator will react with the free and/or residual amines or the base to autonomically indicate the area in which the coating has been damaged.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 62/263,401, filed Dec. 4, 2015, whichis incorporated herein by reference.

BACKGROUND OF THE INVENTION

Polymeric coatings and composites are commonly used to protect asubstrate from wear and environmental exposure. They can be seen inarticles of manufacture in the defense, construction, automobile,aerospace and petroleum industries, just to name a few of them. They areparticularly useful where high strength, stiffness, low weight, andenvironmental stability are required. The coatings can be primers,mid-coats or top-coats. Modern coatings are often formulated to includespecialized functional or multifunctional agents to improve or providevarious different properties of the coating (e.g., indicators of damage,hydrophobicity, ice repellence, anti-fouling, anti-dusting,anti-corrosion, thermo-protection, self-healing, etc.), and are capableof adapting their properties dynamically to an external stimulus. When afunctionalized coating is able to sense the environment and make adesired response to the stimulus, the general term smart coating isoften assigned to it.

The integrity of a coating can be compromised by many things, such as,fatigue, impact or scratch damage, which can expose the underlyingsubstrate to a corrosive environment. Corrosion reduces the mechanicalperformance of substrates, which results in timely and expensiverepairs. Polymeric coatings are often used to protect a substrate fromcorrosion damage. Polymeric coatings are susceptible to damage in theform of small cracks, which can be difficult to detect. Even on a smallscale, crack damage can significantly compromise the integrity andfunctionality of polymeric materials. On metal substrates with polymercoatings, corrosion and other forms of environmental degradation willgenerally initiate at the damage site, compromising the underlyingsubstrate materials. In fiber reinforced polymer composites, smallimpact damage that is difficult to detect can lead to significantdegradation in mechanical performance.

In view of the advances and increased uses of smart coatings, theimportance of damage indication has garnered increased importance.Numerous approaches have been studied to indicate damage in polymericcoatings and biomolecules. A wide range of mechanisms for mechanicallytriggered color change and fluorescence in polymers have been reported,including single molecule turn-on/off fluorescence, mechanochemistry,phase/morphology/defect evolution and embedded capsules. Single moleculeoptical techniques enable force detection at small length scales, suchas studies of cell adhesion and interfaces in biological systems.Biomolecules have also been exploited as mechanophores to revealmicroscopic damage in bulk polymeric composites. However, thesedetection mechanisms are currently restricted to material interfaces andlong-term stability is unknown.

Mechanochemically induced color/fluorescence changes have been generatedunder large strains in bulk polymers. Many of these early mechanophoresexhibit a reversible optical change, and therefore, are not promising todetect permanent damage. A few mechanochemical systems have beendeveloped to indicate damage, but performance has been limited by lowintensity and potential bleaching of fluorescence. U.S. Pat. No.8,846,404 describes a self-indicating polymeric coating wheredamage-induced rupturing of microcapsules initiates a reaction between acharge-transfer donor and a charge-transfer acceptor to form a coloredcharge-transfer product in the damaged area.

Another strategy reported for damage detection is to storecolor-changing indicators in isolated capsules or hollow fibers in apolymer matrix. However, these systems are limited by lack of a turn-onmechanism (e.g., the indicator is always “on”, fluorescent or colored),low contrast between the indicated region and the intact coating, andpoor stability. One reported indication system utilizes two differenttypes of capsules, one type containing crystal violet lactone leuco dyeand the second type containing methyl-4-hydroxybenzoate color developer,embedded in a polymer coating with a solid silica gel color developer.This three-component system produces very low contrast color indicationfor a significant amount of indentation to the coating. The stabilityand controls (e.g., false positives) for this system is unknown.

Damage detection in coatings and composites is challenging. Thedamage-sensing smart coatings described above suffer from significantchemical and mechanical limitations, which make them less desirable touse in many situations. Some of the approaches require human ormechanical intervention, additional components (e.g., color developer orcatalyst), activation (e.g., UV light) associated with significantexternal energy, and possess limited life-spans or are limited to modesttemperatures. A large number of reported indicating systems employcatalysts or other specialty chemicals, which are often expensive,limited to narrow uses and provide less than ideal results in manysituations. Many of these systems are unreliable and limited to specialsituations and conditions. A lot of these systems suffer from one ormore of the following deficiencies: (i) poor color resolution, (iii)lack of versatility (e.g., unstable to certain matrices), (iii) unknownstability, (iv) modest responses and (iv) complicated or expensiveprocessing.

Accordingly, there is a need for improved indicating systems of damagein coatings and composites. New material systems for coatings andcomposites that autonomously indicate the presence of damage and/orother environmental stresses prior to catastrophic failure of thecoating or composite have the potential to decrease costs and enablemore reliable operation in the field. In this patent, we describe anovel system to indicate damage in a material. The system is autonomic,self-powered, stable and adaptable to work on a wide variety of coatingmaterials under different environmental conditions.

SUMMARY

We describe herein a single-component microcapsule-based approach forautonomous detection of damage in polymer coatings and composites. Inone aspect of the invention, an autonomic self-indicating material isprovided comprising a polymer composition having free and/or residualamines and a plurality of first microcapsules having an outer shell andat least a first color indicator encapsulated inside the firstmicrocapsules, where the self-indicating process is autonomicallyinitiated when a region of the material is sufficiently damaged toinduce rupturing of one or more of the first microcapsules, whichrelease the color indicator in and/or around the damaged region where itreacts with the free and/or residual amines in the matrix material toform a colored product in and/or around the damaged region.

In one particular embodiment of the invention, the microcapsule deliveryconcept was demonstrated for an amine-cured epoxy coating thatincorporates an encapsulated reporting agent, 2′,7′-dichlorofluorescein(DCF). The epoxy coating, though cured, possesses free and/or residualamines. Upon the infliction of sufficient damage to an area of thecoating, the microcapsules in and/or around the area of the damage willrupture, and the DCF agent will release into the damaged area andautonomically indicate the area in which the coating has been damaged.An intense, highly localized color change can be seen in the damagedarea.

Another embodiment of the invention is a dual-componentmicrocapsule-based system for autonomous detection of damage in polymercoatings and composites, where one type of microcapsule contains a colorindicator as described herein and a second type of microcapsule isfilled with a base, such as an amine. When the two types ofmicrocapsules rupture due to sufficient damage being inflicted upon anarea of the coating, the color indicator and the amine will each releaseinto the damaged area and react together to autonomically indicate thatthe coating has been damaged. This embodiment is particularly useful forpolymer coatings and composites that do not have sufficient levels offree and/or residual amines to react with the color indicator andprovides autonomous damage indication in a wide range of different typesof polymeric coating materials.

Still another aspect of the invention is a dual-channel vasculardelivery system for autonomous detection of damage in polymer coatingsand composites, where a polymer matrix (comprising materials that do notreact with the indicator) has a vascular network comprising at least twochannels, the first channel comprising a color indicator agent, and thesecond channel comprising a basic agent, such as an amine, where theself-indicating process is autonomically initiated when a region of thematerial is sufficiently damaged, which causes the two channels torelease the color indicator agent and the basic agent in and/or aroundthe damaged region where the two agents react with one another to form acolored product in and/or around the damaged region.

In other embodiments, one or more additional functional agents are addedto the systems described herein. For example, a self-healing agent canbe added to the single-component microcapsule, dual-componentmicrocapsule or dual-channel microvascular delivery system. Whensufficient damage is inflicted on a region of the coating, the injuredregion turns-on a local color change to indicate the presence of thedamage, and the healing agent reacts with the amine (or other hardenersor catalysts that cure the healing agent) to self-repair the damagedregion.

In yet another aspect, a secondary color indicator is added to a systemhaving an additional functional agent, such as a self-healing agent.There can be two types of indicator capsules (or channels) or one typeof indicator capsule (or channel) that possesses two-step color changes.When a region of the coating is sufficiently damaged, a first localcolor change is autonomically turned-on to indicate the presence ofdamage in the region. Then, the damaged region is autonomicallyself-healed and a second local color change is autonomically turned-onindicating the self-healing of the damaged region.

Other embodiments provide methods of autonomically self-indicatingdamage in a material after it has been damaged, comprising formulatingthe single-component microcapsule, dual-component microcapsule ordual-channel vascular delivery system as described herein prior todamage being inflicted upon a region of the material, and observing acolor change in the damaged region after the damage has been inflicted.Further embodiments include formulating an additional functional agent,such as a self-healing agent, and a secondary color indicator, in thesystem and observing the first color indication of the damaged region,the self-healing of the damaged region and the second color indicationthat the damaged region has been repaired.

In one embodiment, the color indicator is DCF. In another embodiment,the color indicator is bromophenol blue (BPB). In yet another embodimentthe color indicator is fluorescamine, which presents blue fluorescenceunder UV light.

The self-reporting ability of the damage color indicator improves safetyand sustainability of materials, enables more reliable operation in thefield and reduces inspection costs as no human intervention is required.Moreover, the damage color indicator can be seamlessly combined with oneor more functional agents, such as self-healing agents and secondarycolor indicators, to report and repair damage to the material, andreport that the material has been repaired.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the specification and are includedto further demonstrate certain embodiments or various aspects of theinvention. In some instances, embodiments of the invention can be bestunderstood by referring to the accompanying drawings in combination withthe detailed description presented herein. The description andaccompanying drawings may highlight a certain specific example, or acertain aspect of the invention. However, one skilled in the art willunderstand that portions of the example or aspect may be used incombination with other examples or aspects of the invention.

FIG. 1 shows a microcapsule with a cutout to view the interior of theouter shell.

FIG. 2 shows an epoxy coating containing indicator microcapsules on topof a steel substrate.

FIG. 3 shows two views of a damaged epoxy coating containing rupturedindicator microcapsules, which color the damaged area.

FIG. 4 shows the chemical reaction and visible spectra of DCF andpolyoxypropylene triamine.

FIG. 5 shows a differential scanning calorimetry graph of the reactionof an epoxy resin and an amine-curing agent subjected to different curecycles.

FIG. 6 shows a mass uptake curve for epoxy specimens soaked in ethylphenyl acetate (EPA) and subjected to different cure cycles, and imagesof a ninhydrin test applied to epoxy specimens subjected to differentcure cycles.

FIG. 7 shows images and visible spectra of the color changes for epoxyspecimens cured under different conditions and soaked in a DCF/EPAsolution.

FIG. 8 shows images of DCF microcapsules before and after beingruptured, and DCF microcapsules immersed in an amine-based curing agentbefore and after being ruptured.

FIG. 9 contains images showing the stability of DCF microcapsules in thepresence of an amine-curing agent.

FIG. 10 shows dynamic and isothermal gravimetric analyses (TGA) of DCFmicrocapsules, and images displaying that the DCF microcapsules remainedintact and active after the isothermal treatment.

FIG. 11 shows DCF microcapsule autonomous damage indication in scratchedepoxy coatings.

FIG. 12 shows photo and color intensities of scratches at varying depthin an epoxy coating having about a 15 wt % DCF microcapsuleconcentration.

FIG. 13 shows color intensities and optical images of identicalscratches at varying loadings of DCF microcapsules in an epoxy coating.

FIG. 14 shows photos and color intensity profile of an epoxy coatinghaving about 10 wt % DCF microcapsule concentration after the coatingwas scratched and stored for a period of time.

FIG. 15 shows images of DCF microcapsules embedded in amine-cured epoxycoatings that were cured at higher temperatures and subjected to damage.

FIG. 16 shows images of DCF microcapsules embedded in an amine-curedepoxy coating compared to three control experiments in a variety ofmicrocapsule-embedded coatings.

FIG. 17 shows images of control and DCF microcapsule coatings that havebeen subjected to scratch and impact damages.

FIG. 18 shows photos and color intensity profiles of an epoxy coatinghaving about 10 wt % DCF microcapsule concentration before and after thescratched coating was stored for a period of time.

FIG. 19 shows images of control and dual-microcapsule systems.

FIG. 20 shows a schematic of a multi-layered coating system containingan epoxy coating embedded with DCF microcapsules and an image of thesystem after having been subjected to light and deep scratch damage.

FIG. 21 shows a dual-channel microvascular network delivery system foran epoxy coating where one channel contains a DCF solution and the otherchannel contains a liquid amine.

FIG. 22 shows the chemical reaction of BPB and an amine and an image ofa ruptured BPB microcapsule embedded in a coating that has beensubjected to damage.

FIG. 23 shows the chemical reaction of fluorescamine and an amine andimages of damaged coatings and composites containingfluorescamine-containing microcapsules and amine-containingmicrocapsules.

DETAILED DESCRIPTION

In contrast to reported multi-component-systems (e.g., a reaction basedon at least two chemicals used to generate the indicating color), thecolor indicator system described herein is compatible for a variety ofmatrix systems and no catalyst or secondary active component is needed.The color indicator system provides an autonomous in situ indication ofdamage as small as 10 μm in polymer coatings and composites. The colorindicator system provides high indicating resolution (e.g., highcontrast between the ‘on’ and ‘off’ states), rapid response, excellentlong-term stability before and after the damage event, sharp color,little to no false positives or bleaching, versatility and costefficiency.

The indication mechanism is directly responsive to damage inflicted onthe material being monitored. The system is very stable and little to nodegradation was observed for over many months for both damaged andintact areas of polymer coatings. In one particular embodiment of theinvention, the first color indicator is DCF and the polymer compositionis an amine-cured epoxy resin coating. The autonomic color change istriggered by a colorimetric reaction of DCF molecules with free and/orresidual amines present in the epoxy resin coating. No new polymers areformed. The color change starts immediately (or almost immediately) andrapidly attains a peak intensity (e.g., from about ten seconds to aboutthirty minutes). The color change is highly stable and remains in and/oraround the damaged region. Its intensity depends on the size andconcentration of reporter capsules, as well as the size of the damage.Cracks as small as 10 microns can be detected.

One embodiment of the invention provides an autonomic self-indicatingmaterial comprising a polymer composition having free and/or residualamines, one or more film-forming binders and a plurality ofmicrocapsules, the microcapsules comprising at least a first colorindicator encapsulated within a volume defined by an outer shell of themicrocapsules, wherein the self-indicating process is autonomicallyinitiated when a region of the material is sufficiently damaged to causerupturing of one or more of the microcapsules, which release the firstcolor indicator in and/or around the damaged region where it reacts withthe free and/or residual amines to form a colored product in and/oraround the damaged region.

The free and/or residual amines in an amine-cured epoxy resin polymercomposition provide an alkaline environment with which a first colorindicator reacts to form a colored product. Other types of polymercompositions can work, as long as they maintain sufficient free and/orresidual amines or other chemical groups that provide alkalineconditions for reacting with the color indicator. For example,polyethylenimine compositions can provide such an alkaline condition.Accordingly, in another embodiment, the polymer composition comprisesfree and/or residual alkali chemical groups other than amines.

In a dual-component (color indicator/base) microcapsule-based deliverysystem, the first color indicator reacts with a base to form a coloredproduct. In certain embodiments, the base is an amine or aromaticheterocycle (e.g., imidazole). For example, primary, secondary andcyclic (e.g., piperidine) amines are suitable bases. In particularembodiments of the invention, the first color indicator is DCF, BPB orfluorescamine.

In some embodiments, the system further comprises at least oneadditional functional agent (e.g., a self-healing or anti-corrosionagent). For example, representative healing agents can be found in U.S.Publication No. 2015/0137416, which is incorporated herein in itsentirety.

In embodiments having an additional functional agent, the system canfurther comprise a secondary color indicator to indicate that theadditional functional agent has performed its function. In certainembodiments, the secondary color change is not generated by reactionwith free and/or residual amines, but rather, is generated by adifferent mechanism. For example, the secondary color change can begenerated by the consumption of free and/or residual amines or by otherconventional mechanisms. Neutral Red and other conventional dyes can beappropriate secondary color indicators.

Accordingly, like with the single component first color (e.g., DCF)capsule indicating systems described herein, a dual component (e.g.,DCF/amine) capsule indicating system can be combined with other capsulesystems to create multi-functional indicating smart coatings. Forexample, a tri-capsule system can be prepared comprising a self-healingagent (e.g., epoxy resin) capsule, a first color indicator (e.g., DCF)capsule and a base-containing (e.g., amine) capsule. This type of systemallows one to prepare multifunctional coatings that not only indicatecrack damage, but also self-heal the damage. In further embodiments ofthe tri-capsule system, a second color indicator (e.g., Neutral Red)capsule can be added to form a quad-capsule self-reporting system havingthe ability to autonomously indicate a damage event, heal the damagedarea, and provide a secondary indication that the damage had beenhealed. In another case, the second color indicator need not besequestered in a separate capsule, but instead, can be loaded withineither the base-containing capsules, the healing agent-containingcapsules or the first color indicator-containing capsules. Similarly,the first color indicator can be encapsulated together with theself-healing agent. The self-healing process can be realized bypolymerization of released healing agents, for example, by adual-capsule system (e.g., epoxy capsules and amine capsules) or asingle capsule system (e.g., epoxy capsules with matrix embeddedhardeners/catalysts). In the single capsule system, the base-containingcapsules can be replaced by embedding hardeners or other types ofcatalysts directly into the matrix materials. A skilled artisan canreadily envision other types of multi-capsule indicating systems andother combinations of two or more agents encapsulated in one type ofcapsule. The requirements for such combinations are that the agents (i)are compatible with one another, (ii) do not react within the capsuleand (iii) remain active to perform their respective functions when adamage event has occurred.

The autonomous visual indication concept is illustrated in the indicatormicrocapsule shown in FIG. 1. Part of the capsule wall (outer shell) 10is cut away to show the indicator agent 15 contained therein. Thecapsule wall protects the encapsulated agent 15 from a matrix to whichit is applied and prior to the capsule being ruptured. When the matrixhas been sufficiently damaged (e.g., by fatigue, impact or scratching),the microcapsule ruptures. The indicator agent is released (andde-protected) upon rupturing of the capsule.

FIG. 2 shows indicator microcapsules 20 being formulated into a polymercoating 25 and applied onto a substrate 30 before damage has beeninflicted to the material. The microcapsules 20 are dispersed throughoutthe polymer coating 25.

After sufficient (e.g., mechanical) damage has been inflicted to aregion of a material, a crack will form in the damaged region and themicrocapsules will be ruptured and release the encapsulated agent(s) inthe damaged region. This aspect is illustrated in FIG. 3. Microcapsulescontaining visual indicators are homogenously dispersed (or embedded) ina polymer coating on a substrate. Upon the infliction of sufficientdamage to a region of the coating, one or more of the capsules rupturein and/or near the damaged region and release the visual indicator(s)contained therein where they chemically react with free and/or residualamines present in the coating (or with a base contained in otherruptured capsules) to locally change color in the damaged area 35 asshown in (a) of FIG. 3. We have shown that microcapsules containing DCFvisual indicators dissolved in ethyl phenyl acetate (EPA) can behomogeneously dispersed in an epoxy coating on a substrate. Whensufficient damage (e.g., scratch, abrasion or compression) is inflictedto a region of the coating, it induces the microcapsules to rupture andrelease the DCF solution in and/or near the damaged region, and adramatic color change from light yellow to bright red is generated inand/or around the damaged region 40 where the DCF species are releasedand come in contact with the polymer coating as shown in (b) of FIG. 3.This figure shows intact capsules a distance away from the damagedregion and ruptured capsules proximate to the damaged region. When thecapsule ruptures, the light yellow colored DCF species reacts with freeand/or residual amines present in the coating (or with a base containedin other ruptured capsules) to produce a dark red colored product in thedamaged region.

A dramatic color change from light yellow to bright red is generatedwhen a DCF species comes in contact with an amine-cured epoxy matrixmaterial. Since DCF provides strong photoluminous signals, only a diluteDCF/EPA solution (e.g., 5 mM) is required. Further, due to a highlylocalized release mechanism and minimal absorption of capsule corematerials into the polymer coating, the delivery of the DCF species ispredominantly restricted to the damaged region, which is advantageousover indicators that flow away from the damaged area.

In embodiments, the wt % amount of color indicator microcapsules withrespect to the total weight of the film or coating is about 0.5-5%,about 5-10%, about 10-15%, about 15-20%, 20-25%, about 25-30%, about30-35%, about 35-40%, about 40-45% or about 45-50%. Amounts greater thana wt % of 50% can also be formulated for use in certain applications.For significant visual indication, a wt % of 5% or greater is generallyneeded for most films or coatings. In a particular embodiment, about 5wt %, 10 wt % or 15 wt % of microcapsules (with respect to the totalweight of the coating) can be added to the epoxy resin film or coating.Since only a little DCF is needed to provide good color indication, theDCF species concentration can be less than about 0.2 wt % or about 0.1wt % with respect to the total weight of the microcapsule.

In certain embodiments, the polymer composition possessing free and/orresidual amines used in the material is based on epoxy resin coatings.The coatings generally have uniform thicknesses, are easy to source andmanufacture and allow for wide applications in industry. Similarly, themicrocapsules are relatively small in size when compared to thethickness of the coating. The capsules can have different sizes thatrange from nanometer to millimeter. In some embodiments, the sizes canbe as small as hundreds of nanometers, which allows for widerapplications.

In some embodiments, the epoxy resin composition comprises bisphenol Aepoxy resin, bisphenol F epoxy resin, novolac epoxy resin, aliphaticepoxy resin, cycloaliphatic epoxy resin and/or glycidylamine epoxyresin. In certain particular embodiments, the epoxy resin compositioncomprises diglycidyl ether of bisphenol A (DGEBA) or diglycidyl ether ofbisphenol F (DGEBF). In a certain embodiment, the epoxy resincomposition comprises an epoxy resin diluted with a low viscosityreactive diluent. In some embodiments, the low viscosity reactivediluent comprises ethyl hexyl glycidyl ether, trimethylol propanetriglycidyl ether, phenyl glycidyl ether or cyclohexane dimethanoldiglycidyl ether. In one embodiment, the low viscosity reactive diluentcomprises o-cresyl glycidyl ether (o-CGE). In one particular embodiment,the bisphenol-A epoxy resin composition comprises EPON 813™ (HEXION).

EPON 813™ (Hexion)

Homo-Polymerized DGEBA

where n denotes the number of polymerized subunits and can number in thetens of thousands or more. In some embodiments, n is in the range from 0to 25, about 1 to about 25, or about 5-20.

As with other classes of thermoset polymer materials, the epoxy resincompositions can be formulated by blending different grades of epoxyresin, and/or adding additives, plasticizers or fillers to achievedesired processing and/or final properties, or to reduce cost. Curingcan be achieved by reacting an epoxy with itself (homo-polymerization)or by forming a co-polymer with polyfunctional curatives or hardeners.In principle, any molecule containing a reactive hydrogen may react withthe epoxide groups of the epoxy resin. Common classes of hardeners forepoxy resins include amines, acids, acid anhydrides, phenols, alcoholsand thiols. Relative reactivity (lowest first) is approximately in theorder: phenol<anhydride<aromatic amine<cycloaliphatic amine<aliphaticamine<thiol. The epoxy curing reaction may be accelerated by addition ofsmall quantities of accelerators. Tertiary amines, carboxylic acids andalcohols (especially phenols) are effective accelerators. Bisphenol A isa highly effective and widely used accelerator.

In some embodiments, the polymeric material comprises an epoxy resincomposition and a catalyst, such as a photo-polymerization catalyst. Incertain embodiments, the photo-polymerization catalyst comprises acationic photoinitiator. In some embodiments, the photo-polymerizationcatalyst comprises IRGACURE® 250 (Iodonium,(4-methylphenyl)[4-(2-methylpropyl) phenyl]-, hexafluorophosphate)(BASF), THP (triarylsulfonium hexafluorophosphate salts)(SIGMA-ALDRICH), THA (triarylsulfonium hexafluoroantimonate salts)(SIGMA-ALDRICH) or DARACUR® 1173(2-hydroxy-2-methyl-1-phenylpropan-1-one (CIBA).

The damage indication will not occur until an area of the substrate issufficiently damaged so as to break open the outer shell wall of themicrocapsules to locally release the encapsulated indicator agentincorporated therein. The outer shell (e.g., polymer shell-wall)provides protection (e.g., thermal and low impact stability) fromunintended rupturing. Accordingly, the outer shell can be a single wallor a multi-walled (e.g., double wall) shell.

The microcapsules can be mixed in an epoxy resin film. In someembodiments, the epoxy resin film comprises zinc-pigmented epoxies,water-based epoxies or DGEBA-based resins. In certain embodiments, theepoxy resin film comprises EPI-REZ™ 6520-WH-53 resin (HEXION) andEPIKURE™ 6870-W-53 (HEXION) curing agent.

The substrate can be anything designed to carry a load, such as astructural or non-structural (e.g., elastomer) substrate. A structuralsubstrate is one that carries the load with minimal deflection.Structural substrates include metal, non-metal, ceramic and polymericmaterials. In certain embodiments, the structural substrate comprises apolymeric structural composite (PMC). PMCs are composed of highstrength/stiffness fibers, held together by a polymer matrix material.Common components include carbon fiber, glass fiber, and epoxy resin. Inone embodiment, the PMC comprises a combination of an epoxy resin withglass or carbon fiber. The epoxy resin can be vacuum infused into theglass or carbon fiber to create a glass or carbon fiber reinforced epoxycomposite. In other embodiments, the structural substrate is steel.

DEFINITIONS

The following definitions are included to provide a clear and consistentunderstanding of the specification and claims. As used herein, therecited terms have the following meanings. All other terms and phrasesused in this specification have their ordinary meanings as one of skillin the art would understand. Such ordinary meanings may be obtained byreference to technical dictionaries.

References in the specification to “one embodiment”, “an embodiment”,etc., indicate that the embodiment described may include a particularaspect, feature, structure, or characteristic, but not every embodimentnecessarily includes that aspect, feature, structure, or characteristic.Moreover, such phrases may, but do not necessarily, refer to the sameembodiment referred to in other portions of the specification. Further,when a particular aspect, feature, structure, or characteristic isdescribed in connection with an embodiment, it is within the knowledgeof one skilled in the art to affect or connect such aspect, feature,structure, or characteristic with other embodiments, whether or notexplicitly described.

The singular forms “a,” “an,” and “the” include plural reference unlessthe context clearly dictates otherwise.

The claims may be drafted to exclude any optional element. Thisstatement is intended to serve as antecedent basis for the use ofexclusive terminology, such as “solely,” “only,” and the like, inconnection with any element described herein, and/or the recitation ofclaim elements or use of “negative” limitations.

The term “and/or” means any one of the items, any combination of theitems, or all of the items with which this term is associated. Thephrases “at least one” and “one or more” are readily understood by oneof skill in the art, particularly when read in context of its usage.

The term “about” can refer to a variation of 5%, ±10%, ±20%, or ±25% ofthe value specified. For example, “about 50” percent can in someembodiments carry a variation from 45 to 55 percent. For integer ranges,the term “about” can include one or two integers greater than and/orless than a recited integer at each end of the range. Unless indicatedotherwise herein, the term “about” is intended to include values (e.g.,numbers recited in weight percentages and material sizes) proximate tothe recited range that are equivalent in terms of the functionality ofthe individual ingredient, material, composition, or embodiment. Theterm about can also modify the end-points of a recited range asdiscussed above in this paragraph.

As will be understood by the skilled artisan, all numbers, includingthose expressing sizes of materials, quantities of ingredients, andproperties, such as molecular weight, reaction conditions, and so forth,are approximations and are understood as being optionally modified inall instances by the term “about.” These values can vary depending uponthe desired properties sought to be obtained by those skilled in the artutilizing the teachings of the descriptions herein. It is alsounderstood that such values inherently contain variability necessarilyresulting from the standard deviations found in their respective testingmeasurements.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges recited herein also encompass any and all possible sub-ranges andcombinations of sub-ranges thereof, as well as the individual valuesmaking up the range, particularly integer values. A recited rangeincludes each specific value, integer, decimal, or identity within therange. Any listed range can be easily recognized as sufficientlydescribing and enabling the same range being broken down into at leastequal halves, thirds, quarters, fifths, or tenths. As a non-limitingexample, each range discussed herein can be readily broken down into alower third, middle third and upper third, etc. As will also beunderstood by one skilled in the art, all language such as “up to”, “atleast”, “greater than”, “less than”, “more than”, “or more”, and thelike, include the number recited and such terms refer to ranges that canbe subsequently broken down into sub-ranges as discussed above. In thesame manner, all ratios recited herein also include all sub-ratiosfalling within the broader ratio. Accordingly, specific values recitedherein are for illustration only and do not exclude other defined valuesor other values within defined ranges.

One skilled in the art will also readily recognize that where membersare grouped together in a common manner, such as in a Markush group, theinvention encompasses not only the entire group listed as a whole, buteach member of the group individually and all possible subgroups of themain group. Additionally, for all purposes, the invention encompassesnot only the main group, but also the main group absent one or more ofthe group members. The invention therefore envisages the explicitexclusion of any one or more of members of a recited group. Accordingly,provisos may apply to any of the disclosed categories or embodimentswhereby any one or more of the recited elements, species, orembodiments, may be excluded from such categories or embodiments, forexample, for use in an explicit negative limitation.

The term “polymer” means a large molecule, or macromolecule, composed ofmany repeated subunits, from which originates a characteristic of highrelative molecular mass and attendant properties. The term “prepolymer”means a precursor containing intermediates or motifs that can undergofurther reaction to form a polymer.

The term “capsule” means a closed object having a shell-wall enclosingan interior volume that may contain a solid, liquid, gas or acombination thereof, and having an aspect ratio of about 1:1 to about1:10. The aspect ratio of an object is the ratio of the shortest axis tothe longest axis, where the axes need not be perpendicular. A capsulemay have any shape that falls within this aspect ratio, such as asphere, a toroid or an irregular amoeboid shape. The surface of acapsule may have any texture, for example, rough or smooth.

Capsules may be made by a variety of techniques and from a variety ofmaterials. Examples of materials from which the capsule shell materialsmay be made, and the techniques for making capsules include:poly(urea-formaldehyde), polyurethane, and polyurea formed byinterfacial polymerization; polystyrene, polydimethylsiloxane, andpoly(phthalaldehyde) formed by solvent evaporation; and all of thesecapsules prepared by a microfluidic approach.

The phrase “autonomic self-indicating material” refers to automatically(without human or electronic control intervention) stopping, startingand adapting operation of the self-indicating material depending onenvironmental or physical stimuli. The objective of the self-indicatingmaterial is to indicate the environmentally or physically damaged regionof the material without any human or machine involvement. For example,when environmental or physical stimuli damage the material so as torupture the described microcapsules contained therein, the capsule corecomposition(s) releases in and/or around the area that is damaged, andautonomic indication of the damaged region can be achieved. Similarly,the material can also be designed to autonomically heal the damagedregion and autonomically indicate that the damaged region has beenhealed.

An “effective amount” refers to an amount effective to bring about arecited effect, such as an amount necessary to form products in areaction mixture. Determination of an effective amount is typicallywithin the capacity of persons skilled in the art, especially in lightof the detailed disclosure provided herein. The term “effective amount”is intended to include an amount of a compound or reagent describedherein, or an amount of a combination of compounds or reagents describedherein, e.g., that is effective to form products in a reaction mixture.Thus, an “effective amount” generally means an amount that provides thedesired effect.

A “sufficient” amount refers to an amount sufficient to bring about arecited effect, such as an amount of damage inflicted to a coating thatis necessary to rupture one or more microcapsules, thereby releasing theagents contained therein. Thus, a “sufficient” amount generally means anamount that provides the desired effect.

The design and operation of an autonomic self-indicating system wasdemonstrated in the following Examples. The system was capable ofself-indicating damage when a sufficient amount of damage was inflictedto it so as to break open microcapsules embedded therein. The followingExamples are intended to illustrate the above invention and should notbe construed as to narrow its scope. One skilled in the art will readilyrecognize that the Examples suggest many other ways in which theinvention could be practiced. It should be understood that numerousvariations and modifications can be made while remaining within thescope of the invention.

Examples Example 1—Materials and General Procedures

(i) Materials

2′,7′-Dichlorofluorescein (DCF, Sigma-Aldrich, St. Louis, Mo.) was usedas a visual indicator for mechanical damage. Ethylene maleic anhydridecopolymer (EMA, Zemac-400, average molecular weight=400,000) fromVertellus (Indianapolis, Ind.), urea, ammonium chloride, resorcinol,1-octanol, formaldehyde solution (37 wt % in H₂O), ethyl phenyl acetate(EPA), sodium hydroxide (NaOH) from Sigma-Aldrich, and commercialpolyurethane (PU) prepolymer (Desmodur L 75) from Bayer MaterialSciencewere used for synthesis of microcapsules. EPONT™ resin 813 (74%diglycidyl ether of bisphenol-A and 26% o-cresyl glycidyl ether) andcuring agent EPIKURET™ 3233 (polyoxypropylene triamine)(Miller-Stephenson, Hoston, Tex.) were selected as matrix materials.Ninhydrin (2,2-Dihydroxyindane-1,3-dione) obtained from Sigma-Aldrichwas used to detect the residual amine content in matrix materials.

(ii) Synthesis of Indicating Microcapsules

The microcapsules were prepared by adapting conventional procedures. Forexample, see the procedures described in Caruso, M. M.; Blaiszik, B. J.;Jin, H. H.; Schelkopf, S. R.; Stradley, D. S.; Sottos, N. R.; White, S.R.; and Moore, J. S., Robust, Double-Walled Microcapsules forSelf-Healing Polymeric Materials, ACSAppl. Mater. Interfaces, Vol. 2(4), pp 1195-1199 (2010), which is incorporated herein in its entirety.In one embodiment, polyurethane/poly-(urea-formaldehyde) (PU/UF)double-shell-wall microcapsules containing DCF were synthesized using asingle batch process. The capsules were prepared in a mixer via in situinterfacial polymerization of urea and formaldehyde (UF) in anoil-in-water emulsion. In brief, about 25 mL of about 2.5 wt % EMAaqueous solution was mixed with additions of about 100 mL H₂O, about 2.5g urea, about 0.25 g ammonium chloride, about 0.25 g resorcinol, andabout 2 droplets of 1-octanol. The pH of the mixture was adjusted toabout 3.5 using about 20 wt % NaOH solution. Stable microcapsulescontaining about a 5 mM DCF/EPA solution as the core material (referredto as DCF microcapsules) were fabricated. Under mechanical agitation atabout 800 rpm, the core solution consisting of about 0.12 g DCF, about60 mL EPA, and about 2 g PU was slowly added into the mixture andallowed to emulsify for about 10 min. Afterward, about 6.33 gformaldehyde solution was introduced into the emulsion and theencapsulation was carried out in a water bath at about 55° C. for about4 h with a heating rate of about 1° C./min. The synthesizedmicrocapsules were filtered, gently rinsed with deionized water toremove the excessive surfactant, and sieved to reduce the sizedistribution. Polydispersed microcapsules were produced with diametersranging between about 20-75 μm and average diameter of about 48 μm.Microcapsules containing an amine (EPIKURE™ 3233) were preparedaccording to a conventional vacuum infiltration method.

(iii) Fabrication of Coatings

Epoxy EPON™ 813 and curing agent EPIKURE™ 3233 were mixed bystoichiometry (e,g., weight ratio of about 100:43). Microcapsules wereadded into the mixture at various weight percentages. The well-mixedsuspension was coated on glass slides or steel substrates using amicrometer controlled doctor blade and cured at about 35° C. for about48 h. Additional thermal treatments of about 6 h at about 50° C., about2 h at about 80° C., and up to about 10 h at about 100° C. were appliedto specific samples. The final thickness of the coating was controlledto be about 350 μm±50 μm.

(iv) Capsule and Coating Characterization

Microcapsules and coatings were examined using optical microscopy (LeicaDMR), scanning electron microscopy (SEM, Philips XL30 ESEM-FEG), andstereomicroscopy (Zeiss SteREO Discovery V20 Microscope). Coatings werealso imaged with a Canon EOS 7D camera. Epoxy swelling was evaluated byimmersing samples in EPA and recording weight change after about 1 h ofsoaking time. About 0.02 g ninhydrin (2,2-dihydroxyindane-1,3-dione)dissolved in about 10 mL ethanol solution was used to detect the freeand/or residual amines in epoxies. Specifically, epoxy samples weresoaked in the ninhydrin-ethanol solution for about 1 h, and then held atabout 100° C. for about 15 min. Free and/or residual amine groups werethen indicated by a produced (Ruhemann's) purple color. For comparison,parallel tests were conducted in about 5 mM DCF/EPA solution todemonstrate the ability of DCF to react with the free or residual aminegroups in the epoxy samples. Molecular structures were determined bynuclear magnetic resonance (NMR) spectra (Varian Unity 400 NB, VarianVXR 500, and Varian Unity 500 NB spectrometer). Visible spectra ofDCF/EPA solution and soaked epoxy samples were obtained by UV-Vis-NIRspectrophotometry (Varian/Cary 5 G). Thermal behavior of DCFmicrocapsules and epoxy coatings were characterized by ThermogravimetricAnalysis (TA Instrument Q50) and Differential Scanning Calorimetry (TAInstrument Q20). The heating rate was kept at about 10° C./min and thepurge gas was nitrogen.

Example 2—Characterization of DCF Color Change

DCF is a good indicating agent due to its high reactivity with a varietyof amines to produce a vibrant color change. DCF is soluble in EPA, anon-toxic solvent that has previously been encapsulated. The DCF/EPAsolution is initially light yellow in color, but changes into an opaquered suspension almost immediately after coming into contact with a base,such as an amine. For instance, adding one drop of polyoxypropylenetriamine (EPIKURE™ 3233, Hexion) to a DCF/EPA solution changed thesolution from light yellow (acid form of DCF) to deep red (base form ofDCF). This reaction and images of the DCF/EPA solutions before and afterbeing exposed to an amine are shown in (a) of FIG. 4, while (b) of thefigure shows the visible spectra of the DCF/EPA solution before andafter addition of the EPIKURE™ 3233 curing agent. The molecularstructures of DCF before and after the reaction are shown in (a) of FIG.4 and were determined by nuclear magnetic resonance (NMR) analysis.

2′,7′-dichlorofluorescein

1H NMR (400 MHz, DMSO): δ=6.66 (s, 2H), 6.91 (s, 2H), 7.34 (d, 1H), 7.75(t, 1H), 7.83 (t, 1H), 8.02 (d, 1H), 11.11 (s, 2H). 13C NMR (500 MHz,DMSO): δ=82.19, 104.38, 111.12, 116.92, 124.62, 125.74, 126.58, 128.88,131.16, 136.54, 150.76, 152.18, 155.80, 168.95.

2-(2,7-dichloro-6-oxido-3-oxo-3H-xanthen-9-yl)benzoate

1H NMR (400 MHz, DMSO): δ=6.16 (s, 2H), 6.74 (s, 2H), 7.12 (d, 1H), 7.50(m, 2H), 8.01 (d, 1H). 13C NMR (500 MHz, DMSO): δ=103.69, 109.72,127.46, 128.26, 129.28, 129.51, 129.74, 130.62, 133.70, 140.60, 157.03,169.97, 173.64.

The addition of the amine caused DCF molecules to evolve into a basicform and precipitate out of the EPA solution due to a dramatic drop insolubility.

Example 3—Cured Epoxy Specimens

Epoxy specimens were prepared with a stoichiometric ratio of EPON™ Resin813 (bisphenol-A based epoxy resin diluted with cresyl, Hexion) andamine-curing agent EPIKURE™ 3233 (100:43) and the specimens weresubjected to various cure cycles. FIG. 5 shows differential scanningcalorimetry of stoichiometric EPON 813 and EPIKURE 3233 with differentcure cycles. The degree of cure was about 96% after about 48 h at about35° C. and about 100% after about an additional approximately 2 h atabout 100° C. Note that the thermal conditions indicated in the graphrefer to the last isothermal process, e.g., those cured at about 100° C.experienced about 48 h at about 35° C., about 6 h at about 50° C., andabout 2 h at about 80° C., prior to the last thermal process.

Example 4—Characterization of Cured Epoxy Specimens

Cured epoxy specimens were soaked in EPA solvent and weighed. A curve ofthe mass uptake of the epoxy specimens (due to EPA swelling) subjectedto different cure cycles is shown in (a) of FIG. 6. The isotherm periodfor cure (1) was about 35° C. for about 48 h, and for cure (2) was about48 h at about 35° C., about 6 h at about 50° C., about 2 h at about 80°C., and 2-10 h at about 100° C. The mass uptake of EPA by the epoxyspecimen after about 1 h soaking was about 2.9% for specimens cured atabout 35° C., and about 1.4% for specimens cured at about 48 h at about35° C., about 6 h at about 50° C., about 2 h at about 80° C., and about100° C. for 10 h. The absorption of EPA into epoxy aids the transport ofDCF species into the matrix and the formation of red precipitates in theepoxy.

We have shown the presence of unreacted (free and/or residual) aminesafter the reaction of EPON™ 813 and EPIKURE™ 3233. The free and/orresidual amines in a cured epoxy coating were detected by a ninhydrin(2,2-dihydroxyindane-1,3-dione) reaction. Amine-cured epoxy specimenswere prepared as described above and imaged after being soaked in aninhydrin-ethanol solution for about 1 h and then heated at about 100°C. for about 15 min. A strong purple color was observed in both samplescured at about 35° C. and about 100° C., as shown in the accompanyingphotos in (b) of FIG. 6. This Ruhemann's purple color was produced inboth tests, which indicates the existence of unreacted amines in theepoxy matrices. Accordingly, even though the epoxies were essentiallyfully cured, (free and/or residual), unreacted amine groups were stillpresent in the matrix.

Example 5—DCF Reaction with Unreacted Amines in a Cured Epoxy Specimen

To examine the ability of DCF to react with free and/or residual amines,cured epoxy samples were soaked in a core solution (e.g., 5 mM DCF/EPAsolution). In (a) and (b) of FIG. 7, the color changes are shown forepoxy specimens soaked in a DCF/EPA solution and cured under differentconditions. DCF precipitates were continuously generated and the entireepoxy specimen turned red when lifted from the solution after about 1 has shown in the images in (a) of FIG. 7. Fracture surfaces of the epoxyspecimens were examined to exclude any possible surface effect, and thereacted red color species were found not only on the surface of theepoxy pieces, but also within the interior of the samples. The curingconditions were (i) about 48 h at about 35° C. for the 35° C. curedepoxy, and (ii) about 48 h at about 35° C., about 6 h at about 50° C.,about 2 h at about 80° C., and finally about 2 h at about 100° C., forthe 100° C. cured epoxy. The scale bar is about 2 mm. Visible spectra ofthe epoxy samples are displayed in (b) of FIG. 7.

In (c) of FIG. 7, the DCF/EPA core solution color changes triggered bythe addition of liquid EPIKURE 3233 curing agent are shown. The colorreaction occurs at a concentration of curing agent as low as 100:1 molarratio of DCF to EPIKURE 3233. For an epoxy specimen prepared with astoichiometric ratio of EPON813-EPIKURE3233 (100:43), the color changeof a 5 mM DCF/EPA solution can be triggered when an estimated 0.0067%amine remained after the curing cycle.

The color changes triggered by residual amines in the epoxy matrix areshown in (d) and (e) of FIG. 7. In (d) of the figure, as-prepared epoxyspecimens cured by various amounts of curing agent (EPIKURE 3233) areshown. From the left to the right, the curing agent concentrationcontinuously decreased from the stoichiometric value to one-third ofthat value. At very low curing agent concentrations, the epoxy cannotfully cross-link to form a solid sample. In (e) of the figure, epoxyspecimens imaged after being soaked in the DCF/EPA solution for about 30minutes are shown. All of the specimens became red in color, indicatingsufficient residual amines for reaction regardless of the initial curingagent concentration.

Example 6—DCF Color Change of Ruptured Microcapsules in the Presence ofan Amine

FIG. 8 shows DCF filled microcapsules before and after being ruptured.Representative SEM images of dry capsules before and after rupture (by arazor blade) are shown in (a) of FIG. 8. The microcapsules are 48±13 μmin diameter and the shell wall was well-formed with a characteristicrough surface and thickness of approximately 300 nm. Capsules were alsoimmersed in an amine-curing agent (e.g., polyoxypropylene triamine) andsome of these capsules were ruptured in a similar fashion as above,which is shown by the optical images in (b) of FIG. 8. The damagedmicrocapsules soaked in an amine-based curing agent immediately changedcolor, while intact microcapsules soaked in the same amine-based curingagent remained unchanged. The color developed when the core materialsencapsulated in the microcapsules were released by rupture of thecapsule shell wall.

Example 7—DCF Microcapsule Stability Studies

The thermal stability of DCF microcapsules in the presence of anamine-curing agent was examined by immersing the DCF microcapsules inEPIKURE™ 3233 for about 48 h. The results are shown in the images ofFIG. 9. In (a) of FIG. 9, the microcapsules survived in the curing agentlong enough for the epoxy resin to fully cure, and remained active toturn on the indication when crushed to release DCF into the curingagent. The control capsules used EPA as the core material. In (b) and(c) of FIG. 9, stereomicroscopic images of DCF microcapsules immersed ina curing agent for (b) 1 min and (c) 48 h are shown. No color changeswere observed and all of the capsules remained intact.

The thermal stability of DCF microcapsules was also evaluated by dynamicand isothermal gravimetric analyses (TGA) at about 120° C. for about 200min. The results are shown in FIG. 10. In (a) of FIG. 10, the dynamicTGA curve of DCF microcapsules at a heating rate of about 10° C./min isshown, while (b) of the figure shows images revealing the isothermal TGAof microcapsules at about 120° C. for about 200 min. Inserts (photos)were taken of the microcapsules immersed in curing agent after thethermal treatment. The microcapsules exhibited excellent thermalstability, remained intact and active after isothermal treatment, andexhibited color change when crushed (right). The scale bars in theinserts are each about 1 cm. Stereomicroscopy images of DCFmicrocapsules (c) before and (d) after thermal treatment at about 120°C. for about 200 min are shown in FIG. 10. No color changes wereobserved and all of the capsules remained intact. In all the tests, theDCF microcapsules remained intact and no color changes or significantweight loss were observed. After each of these stability tests, themicrocapsules were crushed within amine and found to remain activeindicators.

Example 8—Characterization of DCF Color Change in Scratched EpoxySpecimens

FIG. 11 shows autonomous damage indication in scratched epoxy coatingsthat are embedded with DCF microcapsules. DCF capsules were mixed withepoxy and coated (about 350 μm thick) onto a steel substrate. Coatingswere scribed with a stylus and imaged by digital camera (Canon EOS 7D)and stereomicroscope (Zeiss SteREO Discovery V20 Microscope). Thescribed region immediately changed color and the intensity grew strongerand stabilized within a period of about 30 min. Optical images of acontrol epoxy coating having no capsules (left) and an epoxy coatinghaving about 10 wt % of DCF microcapsules (right) on steel substrates,with each coating having an identical scratch, are shown in (a) of FIG.11. The scratch in the self-reporting coating was highly visible incomparison to a significantly less visible identical scratch in thecontrol epoxy coating that did not contain DCF microcapsules. In orderto better evaluate the performance of DCF epoxy coatings and maximizecolor intensity, we carried out experiments on transparent glasssubstrates. Shown in (b) of FIG. 11, are a series of scratches withincreasing depth created in an epoxy coating specimen having about 15 wt% of DCF microcapsules. Looking from left to right in the figure,scratch depth is about 75 μm, about 110 μm, about 150 μm, about 240 μm,and about 370 μm. As the cutting depth was increased, considerably morecapsules were damaged and the color intensity was significantlyenhanced.

The width of the scratches increased with cutting depth, which led torupture of a greater number of microcapsules. In this experiment,scratches as small as about 10 μm in width in the coating were clearlyindicated. Further improvement in indication resolution can be achievedby utilizing smaller microcapsules, though at some point in reducingsize, the microcapsules would require magnified optical observationswhen the current self-reporting ability reaches the limit ofnon-equipment-aided visual detection. The photo and color intensities ofscratches at varying depth in an epoxy coating with about 15 wt % DCFmicrocapsule concentration are shown in FIG. 12. As can be readily seenin the figure, the depths of the scratches continuously increase fromthe left side to the right side.

Color intensity is also dependent on the concentration of DCFmicrocapsules in the coating as shown in (a) of FIG. 13. Identicalscratches were inflicted on epoxy coatings having different DCFmicrocapsule concentrations. Color intensity increased with increasingmicrocapsule concentration due to a higher number density per unit areaof ruptured microcapsules for identical sized scratches, as can bereadily seen in the optical images shown in (b) of FIG. 13. The scalebar in the figure is about 2 mm. In one embodiment, a minimummicrocapsule concentration of about 5 wt % provided sufficient colorintensity for significant visual indication.

FIG. 14 shows photo and color intensities of scratches in an epoxycoating with about 10 wt % DCF microcapsule concentration after thecoating was scratched, imaged and stored for over about 8 months at roomtemperature before being scratched and imaged again. As can be readilyseen in the figure, the depths of the scratches continuously increasefrom the left side to the right side. Little to no change in colorintensity was observed in either the damaged or intact region. Newscratches were then made to undamaged locations, and the autonomousindication was essentially equivalent in intensity to the experimentscarried out about 8 months previously.

We also examined coatings that were cured at higher temperature (about10 h at about 100° C.) and found little to no evidence of thermaldegradation of indication performance. The results are shown in FIG. 15,where (a) shows an epoxy coating with about 10 wt % DCF microcapsulescured at about 35° C. for about 48 h, about 50° C. for about 6 h, about80° C. for about 2 h, and about 100° C. for about 10 h, and (b) showsthe autonomous damage indication in the same coating (scratchedlettering of “ILLINOIS” created by razor blade).

Example 9—Control Coatings with Different Capsules and Coating Materials

Control experiments in a variety of microcapsule-embedded coatingsconfirmed the proposed indicating mechanism. The experiments wereperformed in coatings with (1) non-indicating microcapsules (EPA core)in amine-cured epoxy specimens and (2) DCF microcapsules in amine-freepolymers (e.g., polydimethylsiloxane (PDMS), polyurethane (PU)). Opticalimages of the results are shown in FIG. 16 for three control coatings(a, b, and c), and one targeted embodiment (d), where, (a) is an epoxycoating with non-indicating microcapsules (EPA only as the corematerial), (b) is a PDMS coating with DCF microcapsules, (c) is a PUcoating with DCF microcapsules, and (d) is a DCF indicating coatingsystem comprising epoxy resin EPON™ 813, amine-curing agent EPIKURE™3233, and DCF indicating microcapsules. The microcapsules were kept atabout 10 wt % for each coating, and the scratched regions are labeled inthe photos. As can be readily seen in the images, only the simultaneouspresence of both DCF species and free and/or residual amine groupsprovided autonomous damage indication.

Example 10—Indication Performance of DCF Microcapsules in CommercialPaint

We similarly examined coatings with DCF microcapsules that were added toa gray colored commercial epoxy coating (INTERGARD™ 251 Epoxy Primer,International Paint). FIG. 17 shows optical images of (a) a controlcoating comprising non-indicating microcapsules and (b) DCF indicatingmicrocapsules in the commercial coating matrix. The indicatingperformances of the DCF microcapsules subjected to both scratch andimpact damages were successfully demonstrated. The DCF indicating systemwas highly stable. Scratched coatings were imaged and the colorintensities in all regions were starkly visible compared to the controlcoating.

Example 11—Long-Term Stability Before and after Damage of the SmartCoating

FIG. 18 shows the long-term stability of the autonomous damageindication in DCF microcapsule-embedded epoxy coating. In (a) of thefigure, the photo and color intensity of an epoxy coating stored forover 8 months at room temperature is shown. In (b) of the figure, thephoto and color intensity of an epoxy coating after initial scratchdamage is shown. The DCF microcapsule concentration was 10 wt %. Thedepths of the scratches continuously increase moving from left to right.These results show that the DCF indicating system is highly stable.Scratched coatings stored for over 8 months were imaged again and nochange in color intensity was observed at either damaged or intactregions. New scratches were then made to undamaged locations, and theautonomous indication was equivalent in intensity to those carried out 8months previously.

Example 12—Dual DCF/Amine Microcapsules

In another embodiment, we investigated a dual-microcapsule system forautonomous damage indication, comprising DCF microcapsules andmicrocapsules containing primary amines. The dual-capsule system isuseful for coatings lacking an excess of (unreacted) aminefunctionality. The addition of amine-containing microcapsules enablesdamage indication in coatings without (free and/or residual) unreactedamines available in the matrix polymer (e.g., a non-epoxy coating). FIG.19 shows the damage induced color change in a polydimethylsiloxane(PDMS) coating containing both DCF and amine microcapsules (containingEPIKURE™ 3233 (polyoxypropylene triamine)). FIG. 19 shows (a) about 5 wt% DCF microcapsules in a PDMS coating on a glass slide as a reference,and (b) about 5 wt % DCF microcapsules and about 5 wt % aminemicrocapsules (e.g., dual-microcapsule system) in a PDMS coating on aglass slide. Closer views under microscope of regions 1 a and 2 b in thefigure are provided in (c) and (d), respectively. As can be readily seenin the figure, the dual-capsule system exhibited high indicatingintensity and excellent stability.

Example 13—Damage Indication in a Multi-Layer Coating System

An epoxy coating embedded with DCF capsules was coated on a substrate. Apolyurethane (PU) coating was then coated on top of the epoxy coating toform a multi-layered coating system. The schematic of the coatingstructure is shown in (a) of FIG. 20. The coatings were then subjectedto light and deep scratches. The light scratch only damaged the top PUlayer, and no color change was observed. The deep scratch, however,damaged both the PU and epoxy layers, and the color in the scratchedarea turned to red. Thus, when DCF microcapsules were embedded in thesecond layer from the top, a deep scratch that damaged the top twolayers generated a color change, while a gentle scratch that only wentinto the top layer did not initiate a color change. The light and deepscratches and the color change can be seen in (b) of FIG. 20. Thisexperiment demonstrates that the damage depth can be quantified inmulti-layer coating systems by incorporating the DCF microcapsulesselectively into different layers.

Example 14—Vascular Delivery System

So far, we have exemplified single and dual component microcapsulesystems for delivering a damage-reporting agent. There are other ways todeliver a reporting agent. For instance, another embodiment utilizes adual-channel (micro)vascular network delivery system containing a colorreporting agent and a base liquid (e.g., an amine). In general, apolymer coating or composite can be fabricated with a (micro)vascularnetwork (e.g., polymer matrix has hollow channels to hold liquids) as isillustrated in FIG. 21. Two kinds of liquids (e.g., (i) a colorindicator dissolved in solvent and (ii) an amine) fill two separatechannels. For example, in one embodiment, a DCF/EPA solution fills oneset of channels and an amine-curing agent (e.g., EPIKURE™ 3233) fillsanother set of channels. An undamaged two-channel system isolates onechannel from the other. Then, when mechanical damage is inflicted on anarea of the system, the channels in the damaged area are breached andlocally release the liquids contained therein so that they come incontact with one another in the damaged area. Color reaction isgenerated upon the mixing of the two liquids in the damaged area toindicate the presence of damage. Note that in the DCF indicatorembodiment, the polymer matrix is precluded from having a significantamount of unreacted (free and/or residual) amines to avoid unintendedcolor indication. In other embodiments of the vascular delivery system,additional channels can be added to the (micro)vascular network to holdadditional functional agents, such as self-healing agents and colorreporting agents of self-healing being accomplished. In yet anotherembodiment, a combination of the (micro)vascular network andmicrocapsule systems described herein provide multiple options toautonomically indicate damage, repair damage, and indicate repair hasbeen accomplished.

Example 15—DCF Versus Other pH-Sensitive Dyes

(i) Dye Characterization

DCF is a pH-sensitive dye. The DCF indication mechanism results from achemical reaction under alkaline conditions. Specifically, when the DCFindicator that is encapsulated in microcapsules is released bymechanical damage, the alkaline condition of the environment (e.g.,provided by unreacted amines in a polymeric matrix or amine-filledsecondary microcapsules) initiates a color-changing event.

We tested DCF versus a group of pH-sensitive dyes under the followingtwo criteria: (1) a sharp color change can be triggered by mild alkalineconditions; and (2) the color indicator is oil-soluble andwater-insoluble so that it can be readily encapsulated. In oneembodiment, it is desirable that the indicator color change is triggeredby a mild basic environment provided by amine-cured epoxies. In such acase, no additional additive is required to initiate the indication. Inanother embodiment, it is also desirable that the initial color of theindicator at the encapsulation stage is relatively light to provide aminimum background color and greater contrast to the vibrant color ofthe damage indication. Table 1 contains a list of the dyes tested andthe results.

TABLE 1 Criteria 2 Criteria 1 Suitability for Dye Color Change by AmineEncapsulation 2′,7′- Light Yellow → Bright Red ExcellentDichlorofluorescein (DCF) Bromo Yellow/Green → Blue Excellent phenolblue (BPB) Congo Red No Possible Crystal Violet Lactone No PossibleGenacryl Pink G Purple → Clear Good (not a “turn-on” indication) cannottrigger by residual amine Perylene No Possible Umbelliferone No OkThymol blue Color change cannot be Poor solubility triggered by residualamine Bromocresol purple Color change cannot be Ok triggered by residualamine Bromothymol blue Color change cannot be Ok triggered by residualamine Neutral red Red → yellow Poor oil Questionable to trigger bysolubility residual amine Thymolphthalein Color change cannot beQuestionable triggered by residual amine Fluorescamine No fluorescence →strong blue Excellent fluorescence under UV light

Color changes for the dyes were tested under two alkaline conditions:(1) dye exposure to unreacted (residual and/or free) amines present inan amine-cured epoxy coating (Amine-cured epoxy specimens as describedabove were prepared for testing the dyes of Table 1); and (2) dyeexposure to the addition of a liquid amine. Under criteria 1, ‘no’ meansthat no color change was observed with the addition of a liquid amine,while ‘color change cannot be triggered by residual amine’ means that nocolor change was observed when the dye was exposed to the unreactedamines of a cured epoxy coating, but color change was observed when aliquid amine was added. These findings show that the addition of aliquid amine provides a stronger alkaline environment compared to themild alkaline environment provided by residual and/or free amines of thecoating.

2′,7′-dichlorofluorescein (DCF), bromophenol blue (BPB) andfluorescamine were the only three dyes tested that were suitable fordamage detection based on the above criteria. Genacryl pink G provided acolor change from purple to clear when exposed to a liquid amine. Thisis ‘turn-off’ indication (rather than a ‘turn-on’ indication), whichmeans it would be difficult to observe small ‘color off’ regions withthis dye as an indicator in a strongly colored background. Further,genacryl pink G was not triggered by an unreacted amine of a cured epoxycoating. While neutral red provided a color change upon exposure to anamine, it possessed poor oil solubility and relatively good watersolubility, which means it could not be readily encapsulated. The otherexamined dyes did not satisfy both criteria, and therefore, are not verysuitable for the described microcapsule based systems. For the(micro)vascular delivery system, any vibrant color change that can betriggered by reacting with liquid amines would be suitable forself-indicating.

(ii) BPB Testing

In (a) of FIG. 22, the chemical reaction of BPB and polyoxypropylenetriamine is shown. Like with DCF, BPB is soluble in EPA. A BPB/EPAsolution is initially light yellow in color, but changes into an opaqueblue suspension almost immediately after coming into contact with abase, such as an amine. As can be seen in the images adjacent to themolecular structures of BPB before and after being exposed to anamine-curing agent in (a) of the figure, adding one drop ofpolyoxypropylene triamine (EPIKURE™ 3233, Hexion) to a BPB/EPA solutionchanged the solution from light yellow (acid form) to deep blue (baseform). In (b) of FIG. 22, an image is shown of ruptured BPBmicrocapsules dispersed in a cured epoxy resin that possesses freeand/or residual amines and has been subjected to damage. A vibrant bluecolor can be seen in the damaged region, in contrast to a light yellowcolor seen in the undamaged regions.

(iii) Fluorescamine Testing

Fluorescamine is originally a white, non-fluorescent probe thatselectively becomes irreversibly fluorescent in the presence of primaryamines. Our damage sensing epoxy coating contained a separatelymicroencapsulated amine sensitive fluorescent indicator agent(fluorescamine) in a solvent (e.g. EPA) and an amine-based curing agent(EPIKURE 3233). Once the coating was damaged, the encapsulated indicatorand curing agents release and mix together in the damage region. Whenreacted with a primary amine, fluorescamine reached maximum fluorescentintensity in less than a minute, indicating exactly where the damage hasoccurred.

In (a) of FIG. 23, the chemical reaction of fluorescamine and an amineis shown. Like with DCF and BPB, fluorescamine is soluble in EPA. Asshown in (b) of FIG. 23, a fluorescamine/EPA solution is initially clearand non-fluorescent under UV light, but exhibits bright bluefluorescence under UV light almost immediately after reacting with anamine. Dual microcapsule-embedded epoxy coatings were prepared byincorporating fluorescamine-containing microcapsules andamine-containing microcapsules into an epoxy matrix. In (c) of FIG. 23,photos taken under white light and UV light are shown of damagedcoatings embedded with fluorescamine-containing microcapsules andamine-containing microcapsules. Once the coating has been damaged, theencapsulated indicator and curing agents are released and mixed in thedamage region, generating a vibrant blue fluorescence under UV light.

As shown in (d) of FIG. 23, glass-fiber reinforced composite specimenswith an epoxy coating incorporating fluorescamine microcapsules andamine microcapsules were subjected to impact tests, and the resultingdamage, which was not quite clear under white light, was clearlydiscernible under UV light. Accordingly, other embodiments of theinvention include all the indicator systems exemplified above, exceptthat BPB or fluorescamine is used in place of DCF as the reportingagent.

In summary, the above examples have demonstrated a self-reportingpolymeric coating or composite capable of indicating cracks as small asabout 10 microns in width. DCF, BPB and fluorescamine were successfullyencapsulated and dispersed into several types of polymeric coatings. Inepoxy coatings, the DCF indicator agent released by mechanical damagewas able to react with free and/or residual amines in the coatingmatrix, creating a highly localized red color in the cracked region. TheBPB indicator agent produced a highly localized blue color in thecracked region. The fluorescamine indicator agent exhibited a brightblue fluorescence under UV light after reaction with an amine.

In another embodiment of the invention, through the addition of a secondtype of microcapsule containing a base, such as a primary amine,autonomous damage indication was also achieved in non-epoxy coatings. Inyet another embodiment of the invention, a microvascular delivery systememploying separate channels of color indicator and base can alsoautonomically indicate damage. In all the systems, the indicating colorchange was fast, vibrant, easy to detect and highly stable. Otherembodiments of the invention combine the ability to autonomously detectvirgin damage with self-healing functionality and a secondary indicationthat can reveal that crack healing has occurred. The system canautonomously detect and indicate damage, self-heal the damage, anddetect and indicate that the damage has been healed.

What is claimed is:
 1. An autonomic self-indicating material comprisinga polymer composition having free and/or residual amines and a pluralityof first microcapsules having an outer shell and at least a first colorindicator encapsulated in the first microcapsules, where theself-indicating process is autonomically initiated when a region of thematerial is sufficiently damaged to induce rupturing of one or more ofthe first microcapsules, which release the first color indicator inand/or around the damaged region where it reacts with the free and/orresidual amines to form a first colored product in and/or around thedamaged region.
 2. The material of claim 1 where the material isformulated into a film coated onto a substrate.
 3. The material of claim1 where the first color indicator is DCF, bromophenol blue (BPB) orfluorescamine.
 4. The material of claim 3 where the first colorindicator is DCF.
 5. The material of claim 2 where the substratecomprises a metal, non-metal, ceramic, polymer or fiber-reinforcedcomposite.
 6. The material of claim 2 where the film comprises an epoxyresin.
 7. The material of claim 6 where the epoxy resin comprises a zincpigmented epoxy, water-based epoxy or DGEBA based resin.
 8. The materialof claim 1 where the polymer composition comprises an epoxy resin. 9.The material of claim 8 where the epoxy resin comprises bisphenol Aepoxy resin, bisphenol F epoxy resin, novolac epoxy resin, aliphaticepoxy resin, cycloaliphatic epoxy resin, glycidylamine epoxy resin or acombination thereof.
 10. The material of claim 1 further comprising aplurality of second microcapsules having an outer shell and a functionalagent encapsulated in the second microcapsules, where when theself-indicating process is autonomically initiated, one or more of thesecond microcapsules rupture and release the functional agent in and/oraround the damaged region.
 11. The material of claim 10 where thefunctional agent comprises a self-healing agent.
 12. The material ofclaim 11 further comprising a plurality of third microcapsules having anouter shell and a second color indicator encapsulated in the thirdmicrocapsules, where when the self-indicating process is autonomicallyinitiated, the self-healing agent autonomically heals the damaged regionand one or more of the third microcapsules rupture and release thesecond color indicator in and/or around the healed region where it formsa second colored product different from the first colored product inand/or around the damaged region.
 13. An autonomic self-indicatingmaterial comprising a polymer composition and a plurality of first andsecond microcapsules having an outer shell and one or more agentsencapsulated in each of the first and second microcapsules, the firstmicrocapsule agent comprising a color indicator, and the secondmicrocapsule agent comprising a base, where the self-indicating processis autonomically initiated when a region of the material is sufficientlydamaged to induce rupturing of one or more of the first and secondmicrocapsules, which release the color indicator and the base in and/oraround the damaged region and the two agents react together to form acolored product in and/or around the damaged region.
 14. The material ofclaim 13 where the color indicator is DCF, bromophenol blue (BPB) orfluorescamine and the base is an amine.
 15. An autonomic self-indicatingmaterial comprising a polymer matrix having a vascular networkcomprising at least two channels, the first channel comprising a colorindicator, and the second channel comprising a base, where theself-indicating process is autonomically initiated when a region of thematerial is sufficiently damaged to cause release of the color indicatorand the base in and/or around the damaged region where they react withone another to form a colored product in and/or around the damagedregion.
 16. The material of claim 15 where the color indicator is DCF,bromophenol blue (BPB) or fluorescamine and the base is an amine.
 17. Amethod of autonomically self-indicating damage in a material after ithas been sufficiently damaged, comprising preparing the autonomicself-indicating material of claim 1 prior to the damage being inflictedupon the region of the material, and observing a color change in and/oraround the damaged region after the damage has been inflicted.
 18. Themethod of claim 17 where the color indicator is DCF, bromophenol blue(BPB) or fluorescamine.
 19. A method of autonomically self-indicatingdamage in a material after it has been sufficiently damaged, comprisingpreparing the autonomic self-indicating material of claim 13 prior tothe damage being inflicted upon the region of the material, andobserving a color change in and/or around the damaged region after thedamage has been inflicted.
 20. A method of autonomically self-indicatingdamage in a material after it has been sufficiently damaged, comprisingpreparing the autonomic self-indicating material of claim 15 prior tothe damage being inflicted upon the region of the material, andobserving a color change in and/or around the damaged region after thedamage has been inflicted.